Micro-Electro-Mechanical-Systems (MEMS) sensors are becoming increasingly popular as more sensors are being integrated in a variety of systems ranging from consumer electronics to automobile, aerospace, structural monitoring and life science applications. Although, the majority of the MEMS devices are still in the research and development phase, there is also the mature end of the industry that has been producing large volumes of pressure sensors, microphones, inertial motion sensors and gas sensors. The market for the mature MEMS devices has also been experiencing rapid growth as more applications are being developed with new and innovative packaging designs. Furthermore, new MEMS designs are constantly improving the device performance and integrate more functionality within the same chip, or package with smaller form factor. This has been an enabling factor for MEMS researchers to identify fundamental sensing mechanism and implement them, along with required circuitry, onto silicon chips.
New advances in the field of MEMS have broadened considerably the applications of these devices. MEMS technology has also enabled the miniaturization of the devices; a typical MEMS sensor is at least one order of magnitude smaller compared to a conventional sensor that is used to measure the same parameter. Consequently, MEMS devices can be batch-fabricated, this offers a high potential for unit cost reduction. Moreover, proper design can solve some problems related to power consumption, while providing improved performance characteristics, such as accuracy, sensitivity and resolution.
The current and the traditional MEMS sensor products consist of a sensing mechanical structure, i.e. a cantilever, or a membrane, that deflects upon sensing an external force, i.e. pressure, acoustic waves, or accelerations. The mechanical movement, such as deflection, is then converted into an electric signal through the use of piezo-resistive/electric material, or through changing capacitance between the moving part and a reference plate. There are also various geometries used to enhance the sensitivity and displacement range of these designs that would translate into better sensitivity and dynamic range of the overall sensor.
Optical detection systems based on Fabry-Perot interferometry (FPI) allows for highly sensitive sensors that can potentially detect displacement on sub-nanometer scale. This type of sensor consists of two partially transparent parallel plates with reflective inner surfaces, forming a cavity with an optical resonance that depends on the distance between the plates. At resonant wavelengths, the incident light energy is transmitted through the FPI which forms transmission intensity peaks at the resonant wavelengths. In addition to many applications in optical communications, a wide range of sensors can be based on FPI-principals. This invention discloses a FPI with an actuated mirror that is used as the sensing element of microphones, pressure sensors, gas sensors and accelerometers.
A number of suspended Fabry-Perot optical cavity devices have been developed in the past using micro-fabrication techniques for various applications. Many have been based on integrated bulk micromachining processes due to the stable substrate available for optical deposition. Common to all these devices is that, none were able to achieve a high optical resolution due to mechanical defects or lack of a high quality optical surface on one or both surfaces of the cavity. As of date, no device has been suitable for simple optical serialization for the construction of a distributable array.
However, recent advancements in MEMS fabrication technology and innovations in new designs has overcome the limitations encountered previously in using Fabry-Perot structures for health monitoring sensor arrays: (1) simplified structure, implemented with MEMS technology, enables the fabrication of high finesse Fabry-Perot sensors that have one order of magnitude higher optical sensitivity than previous designs; and (2) A spectrally selective mirror design that enables multiple Fabry-Perot acceleration sensors to be serially concatenated in a simple way. Fabry-Perot (FP) filter for gas analysis in the Mid-Wave Infrared (MWIR) have been fabricated using a new approach that is based on a bulk micro machined Fabry-Perot interferometer with an air cavity, which is electrostatically tuned. Various types of moveable reflectors and spring configurations have been fabricated to determine the optimum solution with respect to maximum tuning range, low gravity influence on center wavelength and suitable filter bandwidth. The main challenge is to achieve a high finesse in spite of non-perfect parallelism, mirror curvature and the additional phase shift caused by the Bragg reflectors. Other approaches to minimizes mirror curvature (hence, higher finesse) by using relatively thick (300 um Silicon) mirror carriers for the coplanar fixed and movable mirrors of the FPI have been reported.
The mirror curvatures can be further minimized by introducing multiple mirror stacks that allows the middle two mirrors to be finely tuned. In four mirror MEMS based FPI the tuning is achieved either by moving one or both middle mirrors. Performance of the FPI is better in terms of spectral range and percentage transmission, when the tuning is achieved by deflecting both middle mirrors in comparison to the performance when single mirror is used for the tuning of the FPI. The spectral range achieved is 2400-4018 nm when tuned with both the middle mirrors and the sensitivity of the FPI is 5.3 nm for every nm deflection in either of the two middle mirrors. FWHM is below 20 nm for most of the tuned positions.